1. Technical Field of the Invention
The present invention generally relates to digital circuits and, in particular, to an apparatus and method for generating a modulated clock signal including harmonics that exhibit a known sideband configuration.
2. Description of Related Art
From the early days of electronic components, size has been an important factor in component design. With the ever increasing integration of electronics into everyday life, component size has become a critical aspect of the design of electronic devices. Although the public is demanding more compact electronics, it is not willing to give up features that add size to those electronics. In fact, the demand for more compact electronic devices is only rivaled by the demand for more features in those devices. Electronic component designers are thus forced to miniaturize electronics while they integrate more features and more components into those electronics.
In no field is miniaturization and integration more important than in the field of circuit design. It is beneficial for a variety of reasons to be able to miniaturize circuits and to place more components in those circuits. Such miniaturization and integration results in, for example, a reduction in manufacturing costs, a reduction in overall device size, and the provision of more features in the same size device. For many electrical devices, especially wireless mobile terminals, it is equally beneficial to integrate digital and analog components. However, the integration of analog and digital components, when combined with ever increasing miniaturization, leads to electromagnetic compatibility (EMC) problems. These EMC problems need to be solved so that highly-integrated systems of digital and analog circuitry can work reliably. For example, in wireless mobile terminals, high-frequency analog signals are handled near digital circuitry which can easily disrupt the high-frequency analog signals and cause degraded receiver performance or spectrum mask violations of transmitted signals.
Presently, EMC issues arising from the integration of digital circuitry and analog circuitry are addressed in many different waysxe2x80x94none of which are wholly satisfactory. Particularly unsatisfactory is the existing digital technology associated with the clock signal which can cause the aforementioned problems if coupled into sensitive analog circuitry. For instance, the harmonic components of the clock signal can fall on a pass band of a radio-frequency (RF) signal and cause interference if that RF signal is coupled to sensitive analog circuitry. Therefore, it is important to control the shape or configuration of the harmonic components of the clock signal.
One traditional way of modifying the configuration of the harmonic components of the digital clock signal is by phase-modulating the digital clock signal. Reference is made to U.S. Pat. Nos. 5,426,392 and 5,731,728 for descriptions of prior art methods associated with phase modulation of clock signals. These traditional phase-modulation methods have drawbacks. In some methods, analog elements and noise sources are used for phase modulation; this practice makes the sideband levels difficult to accurately predict and also complicates testing. In other methods, the digital hardware used to generate the phase-modulated signal is clocked at a very high frequency, which causes high power dissipation in CMOS (Complementary Metal-Oxide Semiconductor) implementations. All the methods referred to are difficult to apply to partial designs, since the phase relationship between signals in different parts would not be constant. Therefore, there is a need for an apparatus and a method capable of generating a phase-modulated clock signal cheaply and predictably, which would also be possible to use on partial designs.
The present invention is an apparatus and method that helps address the electromagnetic compatibility issues arising from the integration of digital circuitry and analog circuitry within a wireless mobile terminal (for example). Basically, the apparatus includes a clock generator capable of generating an unmodulated clock signal and a modulator capable of generating a modulated clock signal having selected clock pulses omitted from the unmodulated clock signal. The modulated clock signal includes a fundamental frequency and a plurality of harmonic frequencies that exhibit a known sideband configuration. The known sideband configuration can be selected such that there would be less harmonic content in a band where co-located analog circuitry could be adversely affected by electro-magnetic interference from the modulated clock signal.